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1

Article type: Research Article1

2

Running head: Preferential water flow through decayed roots of three vegetation types3

4

Title: Preferential water flow through decayed root channels enhances soil water infiltration: Evaluation in5

distinct vegetation types under semi-arid conditions6

7

Authors: Gao-Lin Wu1,2,3,*, Manuel López‐Vicente4, Ze Huang1,2,5, Zeng Cui1,2, Yu Liu1,28

9

Affiliations:10

1 State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and11

Water Conservation, Northwest A & F University, Yangling, Shaanxi 712100, China12

2 Institute of Soil and Water Conservation, Institute of Soil and Water Conservation, Chinese Academy of13

Sciences and Ministry of Water Resource, Yangling, Shaanxi 712100, China14

3 CAS Center for Excellence in Quaternary Science and Global Change, Xi'an, 710061, China15

4 Team Soil, Water and Land Use, Wageningen Environmental Research. Droevendaalsesteeg 3, Wageningen,16

6708RC, Netherlands17

5 College of Natural Resources and Environment, Northwest A & F University, Yangling, Shaanxi 712100,18

China19

20

* Corresponding author: [email protected] (G.L. Wu).21

22

Complete postal address: State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau,23

Northwest A& F University, NO. 26 Xinong Road, Yangling, Shaanxi 712100, P.R. China.24

25

Phone：86-29-87012884 / Fax: 86-29-8701608226

https://doi.org/10.5194/hess-2020-266Preprint. Discussion started: 1 October 2020c© Author(s) 2020. CC BY 4.0 License.

2

Abstract27

Topsoil desiccation alters soil physical characteristics and seriously limits plant growth in semi-arid and arid28

areas. The phenomenon of dried soil layer has generated increasing attention, but the process of preferential29

flow through decayed root channels –when the plants decompose after death– and its benefits on soil water30

supply in the soil dry layers are rarely evaluated. This study examines the effects of root channels on soil31

infiltrability in three contrasted vegetation types developed in a loessial soil, namely: Scrubland (Caragana32

korshinskii), fruit tree plantation (Armeniaca vulgaris) and grassland (Medicago sativa; using data from a33

previous study); setting bare land as control. The infiltration rates of the alive and decayed specimens were34

measured using a double-ring infiltrometer, and methylene blue allowed us to trace the pathways of water35

flow. Results indicated that scrubland species had the highest steady infiltration rates, which were about 23%36

and 83% higher than those rates measured in the fruit tree plantation and grasslands, respectively. Regarding37

root geometry, the steady infiltration rates were significantly and positively correlated with the average root38

channel diameter (ARCD) and area (RCA). Under the same root diameter conditions, soil water infiltrability39

significantly improved in the decayed root plots and compared with the alive root plots. Our findings40

contribute to a better understanding of the effects of root channels of different degraded vegetation types on41

soil moisture and infiltrability, which are conductive to provide knowledge base in the research of42

hydrological processes in degraded soils in water-scarce regions.43

44

Key words: Decayed root; Preferential flow; Double-ring infiltrometer; Soil infiltration; Vegetation type45

46

1. Introduction47

It is expected that droughts are going to be more severe in drylands over the coming years owing to ongoing48

climate change (Brown, 2002; Breshears et al., 2005). Dried soil layers (DSL) are mainly caused by chronic49


3

shortage of rainwater, high evapotranspiration and excessive consumption of deep soil water by plants (Jipp50

et al., 1998). DSL have a negative impact on the water cycle in soil–plant–atmosphere continuum by51

restraining water interchanges between the upper and deeper soil layers (Lucero et al. 2000;52

Valdez-Hernandez et al. 2010). This process may affect the recharge of soil water (Robinson et al., 2006),53

hinders vegetation restoration and may evolve towards soil degradation at a later stage (Ashton and Kelliher,54

1996; Breshears et al., 2005).55

Soil water content is highly dependant on water infiltration and groundwater. Under certain conditions,56

roots create preferential flow paths, and preferential flows can occur via root channels which sometimes57

represents up to 70% of soil macropores (Noguchi et al., 1999). Gerke et al. (2015) found that the biomat58

bacteria layer in the soil can generate preferential flow paths in natural forests. The alive and dead plant roots59

can both form the preferential flow paths that favour water infiltration (Newman et al., 2004; Ghestem et al.,60

2011). Therefore, the macropores formed by root channels are one of the important mechanisms of surface61

water infiltration. Recent studies have indicated that roots have significant effects on soil water infiltrability62

(Wu et al., 2016; Huang et al., 2017), and the traits of plant roots have important effects on the preferential63

flow (Jiang et al., 2018). In particular, the decay of plant roots is related to the vegetation type and root64

density, because the root diameter and length determine the root channel development (Ghestem et al., 2011).65

Wu et al. (2019) studied alfalfa fields and showed that roots decay would improve soil structure and promote66

rainfall infiltration. Roots decay favours restoring soil water conditions, especially by promoting the67

supplement of water in the deep soil layers (Germann et al., 2012). Thus, rational utilization of decayed root68

channels may be an effective way to restore dried soil layers with a minimum environmental impact, and this69

process could provide better soil water conditions for future plants.70

Shrubs are the main plant in drylands (Cushman et al., 2010), and Caragana korshinskii (C. korshinskii) is71

widely cultivated in these regions (Deng et al., 2017) because it is a long-lived legume shrub with good72


4

drought tolerance. However, large-scale planting of C. korshinskii can enhance the soil water deficit and73

further promote the formation of dried soil layers (Gardiol et al., 2003; Maestre et al., 2009). Fruit tree74

plantations are also a common phenomenon in these regions to improve farmers’ income. However and due75

to the limitation of water recharge to the soil and increasing soil desiccation, the mortality of plants has76

increased, which was not conductive to the ecological restoration and economic development.77

As a results of the death of some of these plants, the roots have decayed and formed large channels,78

favouring rainfall infiltration. The soil water condition may improve when rainfall happens, providing a79

better growing environment for the latter plants (Wu et al., 2019). Moreover, a number of studies have80

proved that gramineous plants –with thin and shallow root system– and leguminous plants –with taproot and81

deep root system– have different effects on soil water infiltrability due to their different root architecture82

(Huang et al., 2017; Zhang et al., 2019). Especially different plant types, such as grass, shrubs and trees have83

different effects on soil infiltrability. Although the soil water consumption of trees and shrubs is higher than84

that of grasses (Shangguan, 2007), the effects of root channel formed by decayed roots after vegetation die85

off on soil water reservoir are still not clear. To the best of our knowledge, most studies have concentrated86

the research on one plant type or species (Wu et al., 2016; Jiang et al., 2018; Guo et al., 2019), neglecting the87

comparison between different plant types.88

This study evaluates the effects of decayed and alive roots on soil infiltrability in three contrasted89

vegetation species: Scrubland (C. korshinskii), fruit tree plantation (A. vulgaris) and grassland (Medicago90

sativa; from a previous study made by Guo et al., 2019). This study explores the effects of alive and decayed91

roots on soil water infiltration rates in three common vegetation types in semi-arid regions, and determines92

the contribution of the preferential flow to soil water. This goal fits well with the Land Degradation93

Neutrality target No. 15.3 of the Sustainable Development Goals established by the United Nations (Kapović94

Solomun et al., 2018).95


5

96

2. Materials and methods97

2.1. Study areas98

The experimental site is located in Shanghuang village, Hechuan township, Yuanzhou district, Guyuan city,99

in Ningxia Hui Autonomous Region (35°59′–36°02′N, 106°26′–106°30′E) at an altitude of between100

1530-1822 m a.s.l. This study area is located in the western part of the Loess Plateau, China, and has a101

semi-arid cold climate. The average annual precipitation is ca. 419 mm, but it is widely variable and about102

70% of the total rainfall depth occurs between May and October. The average annual temperature is 6.9 °C.103

The soil in the study area is loessial soil with 26.6% clay, 62.0% silt and 11.4% sand (Chai et al., 2019).104

Since 1980s, a native shrub (Caragana korshinskii) was used to restore the degraded sloping lands in most105

area (ca. 90%). In addition, large areas are cultivated with red plum apricot (Armeniaca vulgaris), which is106

the main economic crop in the area.107

108

2.2. Experimental design109

Three different vegetation types were selected in this study, namely: Scrubland (C. korshinskii), fruit tree110

plantation (A. vulgaris) and grassland (Medicago sativa) (Fig. 1). The data of the grassland species comes111

from a previous study done by Guo et al. (2019). In order to have comparable values between the new and112

old data, we replicated the treatment conditions of the previous study that are the following: Bare land as113

control area, and alive and decayed specimens of C. korshinskii and A. vulgaris. Thirty years ago, C.114

korshinskii was planted in this area because it survives easily in arid and semi-arid environments and115

nowadays it has become the dominant species. However, with the increase of the planting age of the artificial116

C. korshinskii, the plantation has been degraded. The plantation of A. vulgaris started 40 years ago, but some117

plants dead about 5 years ago. The following field measurements experimental design was carried out: Bare118


6

land (4 repetitions), decayed (7 repetitions) and alive (5 repetitions) C. korshinskii, and decayed (8119

repetitions) and alive (6 repetitions) A. vulgaris.120

121

2.3. Measurement of infiltration rates122

Water infiltration processes were evaluated by using a double-ring infiltrometer in the selected sites. This123

device is composed of a 16-cm diameter inner ring and a 32-cm diameter outer ring. It was made by using124

1-cm wall thickness and 20-cm height PVC pipes. For reducing the influence of artificial disturbance on soil125

structure, the infiltrometer were gently and vertically inserted into the soil. The litter and plants were126

removed from the soil surface before inserting the infiltrometer. Then, simultaneous and rapidly addition of127

water to the inner (with methylene blue) and outer (without methylene blue) rings was done up to 5 cm128

height. The time was record each time the water level in the inner ring dropped 1 cm, and measurements129

stopped when they reached three roughly consecutive equal values. Water line in these two rings stayed the130

same, and water was refilled up to the 5 cm height when the water line dropped to 1 cm over the course of131

the experiment. The initial infiltration rate was calculated by the mean value of the first three minutes.132

Similarly, the steady-state infiltration rate was calculated by using the last three values (Fig. 2).133

134

2.4. Soil sampling and analysis135

Vertical soil profiles were excavated along the perimeter of the double ring when the experiment ended after136

24 hours. The vertical soil profiles of the fruit tree plantation (Armeniaca vulgaris), scrubland (Caragana137

korshinskii) and grassland (Medicago sativa) are shown in Fig. 1. The wetted vertical profiles were recorded138

according to the wetted area. Soil bulk density (BD, g cm−3) was obtained by means of using soil bulk139

samplers (100 cm3); at a sampling interval of 10 cm up to 50 cm soil depth. Three replicates were taken at140

each soil layer. Oven-drying method was used to measure soil gravimetric water content (SWC, %). Total141


7

porosity (TP, %) of the soil layers was computed as:142

(1)143

where ds is the soil particle density, and in this study it was 2.65 g cm−3.144

145

2.5. Measurement of root channels146

The root channels diameter (ARCD, cm) was measured by using a vernier caliper (precision of 0.1 mm) that147

allowed us to measure the diameter of the stubbles on the soil surface (Wu et al., 2017; Guo et al., 2019).148

Moreover, the amount of root channels in the inner ring was recorded to reckon the root channel area (RCA,149

cm2). ARCD and RCA were calculated as follows (Wu et al., 2017):150

(2)151

(3)152

where di is the root channel diameter of each root that was measured in the same ring (cm), and n is the sum153

of all root channels that were measured in the same ring.154

155

2.6. Statistical analysis156

The one-way analysis of variance (ANOVA) and the least significant difference (LSD) test were applied to157

examine the differences of the soil characteristics in the same soil layer within the different experimental158

treatments. Significant differences level was set at 0.05. The correlation between ARCD, RCA and the steady159

infiltration rate was evaluated by using regression analysis. All statistical analyses were carried out using160

SPSS 22.0 (IBM, USA) software. All figures were created using SigmaPlot 14.0, except figure 1 that was161

created using Microsoft PowerPoint 2010.162


8

163

3. Results164

3.1. Soil physical characteristics in different treatments165

The highest SWC appeared in the top soil layer (0-10 cm) in all treatments with the highest values in the166

decayed C. korshinskii ( = 16.15%) followed by the bare land ( = 15.83%) and alive C.167

korshinskii ( = 14.13%), whereas the lowest SWC was obtained in the decayed A. vulgaris ( =168

13.36%) (Table 1). The differences of SWC between the treatments were significant at all soil layers. In169

particular, in the 0–40 cm soil layer, the SWC of the decayed C. korshinskii was significantly higher than in170

the other experimental treatments. Moreover, SWC of the bare land was significantly higher than SWC of the171

other treatments at the 40-50 cm soil layer. An overall general trend was observed in all experimental172

treatments, showing that SWC decreased with increasing soil depth. Regarding BD, the measured values173

decreased with increasing soil depth in the bare land, whereas increasing BD values were found in the other174

treatments. The BD of the bare land in the 0-30 cm soil layer was significantly higher ( = 1.24 g cm–3)175

than in the other treatments ( between 1.12 and 1.14 g cm–3). While TP showed an opposite tendency176

compared with BD, the differences between the different soil layers were not marked within the same177

treatment.178

179

3.2. Soil infiltration rates of different treatments180

The initial infiltration rate of the bare land was the lowest (279.07 mm h−1) among all treatments, and it was181

approximately 37%, 57%, 58% and 68% lower than that of the alive and decayed C. korshinskii and A.182

vulgaris, respectively (Fig. 3). Compared with alive and decayed C. korshinskii, the initial infiltration rate of183

the alive and decayed A. vulgaris increased by 48% and 37%, respectively. In addition, the steady infiltration184

rates of decayed C. korshinskii and A. vulgaris were about 68% higher than that of the alive C. korshinskii185


9

and A. vulgaris, respectively. Steady infiltration rates of decayed C. korshinskii and A. vulgaris were186

approximately 83% and 48% higher than that of the bare land, respectively (Fig. 3).187

188

3.3. Correlation between root channel properties and soil water infiltration rates189

The ARCD in the alive and decayed C. korshinskii was positively and significantly correlated to the steady190

infiltration rates (Fig. 4a). Moreover, a significant relationship existed between RCA and the steady191

infiltration rates in the alive (R2 = 0.796; P = 0.108) and decayed (R2 = 0.906; P < 0.001) C. korshinskii (Fig.192

4b). For the alive and decayed A. vulgaris, significant and positive relationships were also found between193

ARCD, RCA and the steady infiltration rates (Fig. 4c, d). Furthermore, the steady infiltration rates of the194

decayed root increased 41% compared to the alive roots when the diameter of the roots were similar in the C.195

korshhinskii (Fig. 4a). The steady infiltration rates of the decayed A. vulgaris increased by 14.21% compared196

with the alive A. vulgaris at the same root diameter (Fig. 4c).197

198

4. Discussion199

Understanding the effects of root characteristics on soil properties is essential to succeed in vegetation200

restoration in drylands (Costantini et al., 2015; Neris et al., 2012). Roots can release large amounts of201

exudates and have a physical winding function that improves soil structure, and further influences soil202

infiltrability (Bronick and Lal, 2005; Wu et al., 2019). Gerke et al. (2015) reported that biomats can transmit203

considerable lateral subsurface flow in some hillslope soils. Moreover, soil preferential flow caused by root204

channels has crucial effects on water infiltration process, which is conductive to the recharge of groundwater205

and has great significance in predicting runoff generation (Weiler and Naef, 2003). Previous studies indicate206

that macropores formed by decayed roots may ease soil water to move down to deep soil layers, especially in207

dried areas (Bogner et al., 2010). Our results provide a clear evidence that the decayed root systems of a208


10

shrub species (Caragana korshinskii) and a fruit tree species (Armeniaca vulgaris) act as preferential flow209

channels for increasing soil water infiltration into soil. The steady infiltration rates of C. korshinskii and A.210

vulgaris significantly and positively correlated with the ARCD and RCA (Fig. 4). The preferential flow211

formed by the root channels is considered to be the main effect factor on macropore flow (Weiler and Naef,212

2003). This process may be relevant for the restoration of the soil water conditions in deep layers, and thus,213

to mitigate dried soil layers.214

This study shows that the steady infiltration rates of both the alive and decayed roots increased compared215

with the bare land, and more important, this pattern was higher in the decayed roots treatment. In general,216

both alive and decayed roots could form relatively large, continuous, well-connected and open channel217

networks, considering preferential flow paths (Aubertin, 1971). Compared with alive roots, decayed roots218

were more likely to form long and continuous paths (Mitchell et al., 1995). However, tree root morphology219

could have an adverse effect on soil slope stability of forested hillslopes, caused by hydrological processes,220

due to the increase of pore water pressure in the sensible soil zone (Sidle, 2000). After aggregating our field221

measurements with the previous research findings (Guo et al., 2019), we found that the infiltrability of222

scrubland species (C. korshinskii) was significantly improved compared with the rates obtained in the fruit223

tree (A. vulgaris) and grassland (Medicago sativa) species. This result may be explained by the coarser roots224

of A. vulgaris than those of C. korshinskii, which led to different decay rates. Moreover, this finding was also225

explained by the dye tracer experiment. The decayed root hole in C. korshinskii contains a blue dye (Fig.1)226

that clearly indicated that the decayed root of C. korshinskii formed a channel to promote water flow, but the227

dye was not found in the decayed root of A. vulgaris (Fig. 1).228

It is worth mentioning that this study proved the positive correlation between ARCD and RCA with229

steady infiltration rates. The ARCD in the leguminous grasslands (M. sativa) and scrublands (C. korshinskii)230

was approximately 3 cm, whereas in the fruit tree plantations (A. vulgaris) it was more than 10 cm. The231


11

relative larger RCA was conductive to improve the soil water steady infiltration rate. Moreover, the232

distribution of roots in the soil profile may lead to the difference of infiltration depth. Results indicated that233

infiltration depth of the scrublands was significantly higher than that of the fruit tree plantations (Fig. 5),234

which may be resulted from the unrotten root system of the orchard. Normally, the root system of fruit trees235

is deeper than that of the scrublands and grasslands. This study has explored the effects of decayed root236

channels on soil hydrology, which may contribute to the sustainability of vegetation restoration in drylands.237

This task is of particular importance in water-scarce areas, and further in-depth study is still needed. Hence,238

future study should pay more attention on preferential flow related to complex root morphologies and239

different decay degrees in different vegetation types.240

241

5. Conclusions242

The effects of root channels on soil water infiltrability of three distinct vegetation types were studied in a243

semi-arid area, and the study was divided into those channels formed by decayed roots or alive roots.244

Combining the author's previous research findings, we concluded that scrubland species had the highest245

value of steady infiltration rates, followed by fruit tree and grassland species. The results of this study clearly246

demonstrated that the root channel formed by decayed roots could improve soil water infiltration rates247

compared with the rates obtained in the alive roots of the same species. Importantly, the steady infiltration248

rates were significantly and positively correlated to the ARCD and RCA. Under the same root diameter249

condition, the steady infiltration rates of the decayed roots were significantly higher than those of the alive250

roots. Our results suggested that the decayed root channels significantly increased the soil infiltrability251

compared with the alive root channels. Our research contributes to better understanding the role played by252

the decayed root channels of different degraded vegetation types on soil infiltrability and soil moisture, and253

thus, to improve the dried soil layer in semi-arid regions. This process of soil water replenishment is254


12

environmentally friendly and matches the target of Land Degradation Neutrality.255

256

Declaration of Competing Interest257

The authors declare no conflict of interest.258

259

Acknowledgements260

This research was funded by the National Natural Science Foundation of China (NSFC 41722107, 41930755,261

41977063), the Youth Talent Plan Foundation of Northwest A & F University (2452018025, 2452018086)262

and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB40020302). We thank263

Lei Guo and Ji-Wei Gao for assistance with field works.264

265

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Valdez-Hernandez, M., Andrade, J. L., Jackson, P. C., Rebolledo-Vieyra, M.: Phenology of five tree species335

of a tropical dry forest in Yucatan, Mexico: effects of environmental and physiological factors. Plant336

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2017.347

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improved mine soil infiltration capacity. J. Hydrol., 535, 54-60, 2016.349

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associated with fibrous and tap tree roots. Water, 11, 1700, 2019.353


17

Table 1. The characteristics of soil water content (SWC), soil bulk density (BD) and total porosity (TP) of354

bare land, Caragana korshinskii and Armeniaca vulgaris at 0–50 cm soil layers (Mean ± SD).355

Treatments Soil depth

(cm)

SWC

(%)

BD

(g cm-3)

TP

(%)

Bare land 0-10 15.83±1.10ab 1.25±0.05a 52.74±1.98b

10-20 11.62±0.21b 1.26±0.02a 52.56±0.74b

20-30 11.10±1.07b 1.22±0.03a 53.93±1.05b

30-40 9.73±0.46b 1.17±0.05a 55.81±1.88a

40-50 10.56±0.56a 1.13±0.01a 57.25±0.40a

Alive Caragana korshinskii 0-10 14.13±0.51b 1.11±0.06b 57.94±2.14a

10-20 13.86±0.14ab 1.13±0.06b 57.26±2.15a

20-30 13.78±0.47ab 1.12±0.03b 57.70±1.14a

30-40 11.69±0.71a 1.13±0.01a 57.41±0.32a

40-50 7.98±0.62c 1.17±0.08a 56.03±3.05a

Decayed Caragana korshinskii 0-10 16.15±0.14a 1.09±0.06b 58.93±2.22a

10-20 14.43±1.14a 1.10±0.07b 58.42±2.64a

20-30 14.84±0.56a 1.16±0.04ab 56.16±1.69a

30-40 11.17±0.40a 1.12±0.05a 57.56±1.99a

40-50 9.21±0.79b 1.17±0.03a 55.84±0.94a

Decayed Armeniaca vulgaris 0-10 13.36±1.44b 1.15±0.06ab 56.50±2.28ab

10-20 12.69±1.60ab 1.08±0.06b 59.17±2.42a

20-30 9.43±2.70b 1.19±0.02a 55.04±0.65b

30-40 6.78±0.37c 1.14±0.07a 56.86±2.78a

40-50 7.23±0.17c 1.21±0.06a 54.40±2.30a

Note: For each soil depth but different treatment, the values followed by a different letter are significantly356

different at 0.05 level.357


18

358

Figure 1. The experimental sites in the fruit tree plantation (Armeniaca vulgaris), scrubland (Caragana359

korshinskii) and grassland (Medicago sativa). This figure showed the roots from alive to decay when the360

plants die off. The light blue area indicated the wetting front when measure the infiltration rate by using361

double-ring infiltrometer. The infiltration depth was measured after the experiment stopped 24 h when the362

infiltration rate was stable.363


19

364

Figure 2. Infiltration processes and cumulative infiltration curves for the bare land, scrubland (Caragana365

korshinskii), fruit tree plantation (Armeniaca vulgaris) and grassland (Medicago sativa).366


20

367

Figure 3. Initial infiltration rate and steady infiltration rate of different experimental treatments (mean ± SD).368

Note: BL, bare land; AC, alive Caragana korshinskii; DC, decayed Caragana korshinskii; AA, alive369

Armeniaca vulgaris; DA, decayed Armeniaca vulgaris.370

371


21

372

Figure 4. Correlation between the average root channel diameter and the root channel area vs. the373

steady-state infiltration rate of decayed and alive scrubland (Caragana korshinskii) and fruit tree (Armeniaca374

vulgaris).375

37640.88% 41.09%

14.21% 14.69%

(a) (b)

(c)(d)


22

377

Figure 5. The maximum soil water infiltration depth of different experimental treatments (mean ± SD). Note:378

The maximum infiltration depth is the maximum depth the water reached along the soil profile after 24 h. BL,379

bare land; AC, alive Caragana korshinskii; DC, decayed Caragana korshinskii; DA, decayed Armeniaca380

vulgaris.381


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